Infiltrating leukocytes play a key role in the pathogenesis of renal injury in SLE. In SLE, leukocytes are recruited to the kidney in response to immune complex (IC) deposition. Current concepts do not, however adequately explain how IC deposition leads to tissue leukocyte infiltration. We postulate that IC interact with leukocytes and renal parenchymal cells to activate local production of chemokines that are then responsible for recruiting inflammatory cells to the kidney. The severity of renal injury in SLE is dependent on endogenous mechanisms that regulate chemokine bioactivity, and on genetic factors that determine the intensity of chemokine expression in response to IC deposition. Aim 1 will test the hypothesis that IC-induced chemokine expression in the kidney is mediated by Fcgamma-receptor III bearing lymphocytes that have been activated by IC. A cell culture model of pathogenic IC interaction with leukocytes and renal was developed for this evaluation. Evidence to support this model will be sought in the SLE nephritis patient population. Aim 2 will investigate endogenous protective mechanisms that may be activated during SLE nephritis to attenuate the biologic activity of chemokines. These putative protective factors include parenchymal chemokine receptors and inhibitory complement components. Chemokine receptors expressed by the kidney during SLE nephritis will be determined by immunohistochemically. Regulation of receptor expression by cultured renal cells will be investigated. In a co- culture system of leukocytes and renal cells, the effects of parenchymal chemokine receptors on chemokine bioactivity will be assessed. Aim 3 will test the hypothesis that polymorphisms in the regulatory regions of chemokine genes affect the chemokine response to activating stimuli. DNA from healthy individuals will be sequenced to identify such polymorphisms. Regulatory region variants will be ligated into a reporter vector for functional evaluation. The presence of functional polymorphisms in the SLE nephritis population will be correlated to severity of renal injury. Finally, with Project 4, urine chemokine levels in SLE nephritis patients will be followed prospectively to determine whether chemokine expression predicts onset or severity of SLE relapse. This project is expected to offer novel insight into the pathogenesis of renal leukocyte infiltration in SLE. These studies will thus address the Program Project question of how defective clearance of pathogenic IC in SLE results in renal injury.